The present invention relates to an acoustic matching member used, when a sound is propagated from one object to another object, for matching acoustic impedances of the two objects, a method for producing the acoustic matching member, and an ultrasonic transmitting and receiving device using the acoustic matching member.
An acoustic impedance of an object is obtained by (densityxc3x97sonic speed). The acoustic impedance of air ZAIR is about 428 kg/m2s, and the acoustic impedance of a piezoelectric vibrator ZPZT for generating an ultrasonic wave is about 30xc3x97106 kg/m2s.
When an ultrasonic wave is radiated from the piezoelectric vibrator into the air, sound reflection is generated by the difference between the acoustic impedance of the piezoelectric vibrator ZPZT and the acoustic impedance of air ZAIR, and thus the radiation efficiency of the sound is reduced.
The acoustic matching member is used to alleviate the reduction in the radiation efficiency of the sound by matching the acoustic impedance of the piezoelectric vibrator ZPZT and the acoustic impedance of air ZAIR.
An acoustic impedance of an acoustic matching member ZM is obtained by expression (1) based on theoretical calculation.
ZM={square root over ( )}(ZPZTxc3x97ZAIR)xe2x80x83xe2x80x83expression (1)
Here, the value of ZM is an ideal value at which there is no sound reflection. Using the above-mentioned values of ZPZT and ZAIR, the value of ZM is about 0.11xc3x97106 kg/m2s.
FIG. 29 is a graph illustrating the relationship between the acoustic impedance of the acoustic matching member and the ratio of the sound energy radiated from the piezoelectric vibrator into the air (transmission ratio). It is appreciated from FIG. 29 that when the acoustic impedance of the acoustic matching member is about 0.11xc3x97106 kg/m2s, the transmission ratio is 1, at which there is no sound reflection.
In order to obtain an acoustic matching member having such an ideal acoustic impedance, a material having a low density and allowing a low sonic speed needs to be selected for such an acoustic matching member.
FIG. 30 shows an example of a conventional acoustic matching member. The acoustic matching member shown in FIG. 30 is obtained by mixing glass balloons 121 in a resin material 120 and solidifying the resultant mixture.
The glass balloons are hollow and thus have a feature of being very lightweight. A structure obtained by mixing the glass balloons in the resin material and solidifying the resultant mixture has a lower density than that of a structure obtained by solidifying only the resin material. The size of the glass balloons is set to a value which is sufficiently smaller than the wavelength of the vibration (sound) propagating through the acoustic matching member (about {fraction (1/10)} of the wavelength of the vibration or less). The size of the glass balloons is set to such a value in order to make the propagation of the vibration less liable to the influence of the glass balloons.
When glass balloons having a true density of 0.13 g/cm3 (xe2x80x9cScotchlight(trademark) Glass Bubbles Fillerxe2x80x9d available from Sumitomo 3M Ltd.) are mixed in a resin material allowing a sonic speed of about 2300 m/s and having a density of 1.2 g/cm3, and the resultant mixture is solidified, a structure having a density of 0.56 g/cm3 and allowing a sonic speed of 2100 m/s is obtained. An acoustic impedance ZCOM of the structure thus obtained is 1.18xc3x97106 kg/m2s.
Japanese Laid-Open Publication No. 2-177799 describes that an acoustic matching member is formed using only hollow glass spheres. This acoustic matching member is produced by heating the hollow spheres up to a temperature for softening the hollow glass spheres, compressing the hollow spheres, and binding the plurality of hollow spheres at respective contact points. As the hollow glass spheres, xe2x80x9cScotchlight(trademark) Glass Bubbles Fillerxe2x80x9d available from Sumitomo 3M Ltd. is used. Japanese Laid-Open Publication No. 2-177799 describes that the acoustic matching member thus produced has characteristics of a sonic speed of 900 m/s and an acoustic impedance ZBG of about 0.45xc3x97106 kg/m2s. Since the acoustic impedance of an object is represented by (sonic speedxc3x97density), the density of this acoustic matching member is 0.5 g/cm3.
As described above, the sonic speed allowed by glass is 5000 to 6000 m/s, but the sonic speed allowed by an acoustic matching member is reduced to 900 m/s by producing the acoustic matching member using hollow glass spheres.
An acoustic matching member can be bonded to a vibrator or a case accommodating the vibrator with an adhesive formed of a resin material such as an epoxy resin. Japanese Laid-Open Publication No. 2-177799 describes an example of heating the plurality of hollow spheres up to a temperature for softening the plurality of hollow spheres and binding the plurality of hollow spheres at respective contact points as well as bonding the acoustic matching member to a vibrator. By such a bonding method, the acoustic matching member is formed only of glass, and thus has superior temperature characteristics to those of an acoustic matching member formed using a resin material. The reason is that the thermal expansion ratio of glass is lower than the thermal expansion ratio of the resin material. When the flow rate of a gas is measured using an ultrasonic transmitting and receiving device, the temperature characteristics of the ultrasonic transmitting and receiving device significantly influences the measurement precision. In order to accurately measure a very small flow rate of gas, the temperature characteristics of the ultrasonic transmitting and receiving device need to be small.
Some types of gases are explosive. A vibrator which needs to provide such a gas with an electric signal is required to be accommodated in a case in order to prevent the vibrator from contacting the gas. Conditions to be satisfied by the material of the case include a high strength against breakage and satisfactory temperature characteristics. For this reason, metal is preferable as a material of the case. The thermal expansion ratio of metal is different from the thermal expansion ratio of glass. Therefore, a metal case and the acoustic matching member come apart from each other and cannot be bonded together at the stage of binding the plurality of hollow spheres at respective contact points after the plurality of hollow spheres are heated to a temperature for softening the plurality of hollow spheres as in the method described in Japanese Laid-Open Publication No. 2-177799.
When the acoustic impedances ZBG and ZCOM of the above-described acoustic matching materials are plotted in the graph of FIG. 29, ZBG is positioned at A and ZCOM is positioned at ▪. The transmission ratio is 0.21 for ZBG and 0.05 for ZCOM. Thus, the transmission ratio (i.e., the transmission ratio of sound) for ZBG is four times the transmission ratio for ZCOM. However, in actuality, an output which is four times larger is not obtained, but the outputs are of an equivalent level for ZBG and ZCOM. This is considered to occur since the structure having ZBG is more likely to cause the sound to attenuate when the sound is propagated therethrough than the structure having ZCOM. By contrast, the structure having ZCOM is less likely to cause the sound to attenuate while the sound is propagated therethrough but allows a higher sonic speed than the structure having ZBG. Therefore, the structure having ZCOM has a larger acoustic impedance and causes the sound radiated into the air to be reflected more than the structure having ZBG.
In the end, there is no significant difference in the sound outputs of the both types of acoustic matching members. Therefore, an acoustic matching member providing a large sound output is demanded rather than the acoustic matching member formed of the structure having ZBG or ZCOM One possible reason why the structure having ZBG causes the sound to significantly attenuate is that because the hollow spheres bound at only the respective contact points and thus the total number of contact points are relatively small.
As described above, the conventional ultrasonic transmitting and receiving device has the following problems.
First, when the acoustic matching member is formed using a resin material, the measurement precision of the ultrasonic transmitting and receiving device is not satisfactory due to the temperature characteristics of the resin material.
Second, when the acoustic matching member is formed of only hollow glass spheres, sound is significantly attenuates due to a small number of contact points of the hollow spheres.
Third, when the vibrator is accommodated in a metal case so as to prevent the vibrator from contacting the gas, bonding of the acoustic matching member to the metal case with an adhesive formed of a resin material such as an epoxy resin deteriorates the measurement precision of the ultrasonic transmitting and receiving device due to the temperature characteristics of the adhesive.
Fourth, the metal case and the acoustic matching member come apart from each other and cannot be bonded together at the stage of binding the plurality of hollow spheres at respective contact points after the plurality of hollow spheres are heated to a temperature for softening the plurality of hollow spheres, due to the difference in thermal expansion ratio of the metal case and glass, which is the material of the hollow spheres. Even when the metal case and the acoustic matching member are bonded, flexure is generated and thus the vibration of the vibrator is not propagated.
The present invention has been made in order to solve the first through fourth problems.
An acoustic matching member according to the present invention is used, when a sound is propagated from a first object to a second object, for matching an acoustic impedance of the first object and an acoustic impedance of the second object. The acoustic matching member includes a plurality of fine pieces. At least one of the plurality of fine pieces is bonded with at least another of the plurality of fine pieces at a contact portion so as to form a gap in the acoustic matching member.
The plurality of fine pieces may each have an amorphous three-dimensional structure.
The plurality of fine pieces may be located so as to prevent the sound from being linearly propagated through the acoustic matching member.
The plurality of fine pieces may each be formed of a glass or a ceramic material.
A method for producing an acoustic matching member according to the present invention is used, when a sound is propagated from a first object to a second object, for matching an acoustic impedance of the first object and an acoustic impedance of the second object. The method includes the steps of (a) forming a plurality of fine pieces; (b) heating the plurality of fine pieces to a temperature for softening the plurality of fine pieces, thereby bonding at least one of the plurality of fine pieces with at least another of the plurality of fine pieces at a contact portion so as to form a gap in the acoustic matching member.
The step (b) may include the step of heating the plurality of fine pieces while applying a load on the plurality of fine pieces.
The step (a) may include the steps of mixing the plurality of fine pieces and a liquid; and vaporizing the liquid from a mixture of the plurality of fine pieces and the liquid.
A specific gravity of the liquid may be smaller than a specific gravity of the plurality of fine pieces.
The liquid may be vaporized after the plurality of fine pieces are precipitated in the liquid.
The plurality of fine pieces may be formed by pulverizing a plurality of hollow spheres.
A density of the acoustic matching member may be controlled in accordance with a degree to which the plurality of hollow spheres are pulverized.
The degree to which the plurality of hollow spheres is pulverized may be expressed by a ratio between a volume of the plurality of hollow spheres before being pulverized and a volume of the plurality of fine pieces obtained by pulverizing the plurality of hollow spheres.
An ultrasonic transmitting and receiving device according to the present invention includes a vibrator; a metal case for accommodating the vibrator; an acoustic matching member used for matching an acoustic impedance of the vibrator and an acoustic impedance of a fluid flowing outside the metal case; and a bonding member for bonding the acoustic matching member and the metal case. The acoustic matching member includes a plurality of fine pieces, and at least one of the plurality of fine pieces is bonded with at least another of the plurality of fine pieces at a contact portion so as to form a gap in the acoustic matching member. The bonding member has a structure for reducing a difference between a thermal expansion ratio of the metal case and a thermal expansion ratio of the acoustic matching member.
The bonding member may include a first layer formed on the metal case, a second layer formed on the first layer, and a third layer formed on the second layer. The first layer may be formed of silver solder. The second layer may be formed of titanium. The third layer may be formed of silver solder.
The bonding member may further include a fourth layer formed on the third layer and a fifth layer formed on the fourth layer. The fourth layer may be a ceramic plate or a glass plate. The fifth layer may be formed of glass having a melting point lower than a melting point of the material of the fourth layer.
The bonding member may include a first layer formed on the metal case, and the first layer may be formed based on a mixture obtained by mixing silver solder powder and titanium powder.
The bonding member may include a first layer formed on the metal case, and the first layer may be formed based on a mixture obtained by mixing silver solder powder, titanium powder and ceramic powder.
The bonding member may include a first layer formed on the metal case and a second layer formed on the first layer, and a bonding face between the first layer and the second layer may have a convexed and concaved shape.
The first layer may be formed on the metal case intermittently.
The first layer may contain a plurality of particles having a thermal expansion ratio lower than a thermal expansion ratio of the material of the first layer.
The bonding member may include a first layer formed on the metal case and a second layer formed on the first layer. The first layer may be formed by heating a mixture containing a first particle of a first material which is easily oxidized, nitrided or carbided and a second particle of a second material having a specific gravity larger than a specific gravity of the first material and having a melting point lower than a melting point of the first material, the first layer being formed as a layer of the second material. The second layer may be formed on the layer of the second material, the second layer being formed as a layer obtained as a result of oxidizing, nitriding or carbiding the first material.
The first material may have a thermal expansion ratio which is lower than a thermal expansion ratio of the second material.
The mixture may be heated at a temperature which is lower than the melting point of the first material and higher than the melting point of the second material.
The first particle may have a size of 150 xcexcm or less.